How to render ultra-realistic looking models and textures in Blender?
If you want to create realistic humans you should first study anatomy. Really study it, learn the bones, the muscles, etc. Learn to draw the human figure.
Then learn the sculpting tools in Blender (or another sculpting software like Zbrush, obviously, but this is a Blender forum so I'll stick to blender).
If you want to work in real productions, also learn retopology. Learn modeling fundamentals. Learning hard surface modeling would be good too.
Learn how to open UVs in an efficient way. Learn how to texture the models. Study how real light behaves and how it bounces off different materials. Understand subsurface scattering (really important for humans).
Learn to create and groom realistic hair and fur.
Learn how to create good lighting, like a photographer or cinematographer would. You also need to learn the render settings and what each setting does.
So basically:

Anatomy and figure drawing
Sculpting in 3D software
3D Modeling fundamentals (organic and hard surface)
Retopology
Opening UVs
Optics (how light behaves and interacts with different materials)
Editing physically based shaders
Hair and fur
Lighting and Render

I think that's about it.
I recommend this series of videos to start studying anatomy: https://youtu.be/pDgyQjNFVQk